.\" Copyright 1993 Giorgio Ciucci .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond .\" Modified Mon Jul 10 21:09:59 2000 by aeb .\" Modified 1 Jun 2002, Michael Kerrisk .\" Language clean-ups. .\" Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX .\" Added note on restart behavior of msgsnd() and msgrcv() .\" Formatting clean-ups (argument and field names marked as .I .\" instead of .B) .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" Modified, 11 Nov 2004, Michael Kerrisk .\" Language and formatting clean-ups .\" Added notes on /proc files .\" FIXME . Add example programs to this page. .\" .TH MSGOP 2 2012-05-31 "Linux" "Linux Programmer's Manual" .SH NAME msgrcv, msgsnd \- message operations .SH SYNOPSIS .nf .B #include .B #include .B #include .sp .BI "int msgsnd(int " msqid ", const void *" msgp ", size_t " msgsz \ ", int " msgflg ); .sp .BI "ssize_t msgrcv(int " msqid ", void *" msgp ", size_t " msgsz \ ", long " msgtyp , .BI " int " msgflg ); .fi .SH DESCRIPTION The .BR msgsnd () and .BR msgrcv () system calls are used, respectively, to send messages to, and receive messages from, a message queue. The calling process must have write permission on the message queue in order to send a message, and read permission to receive a message. .PP The .I msgp argument is a pointer to caller-defined structure of the following general form: .in +4n .nf struct msgbuf { long mtype; /* message type, must be > 0 */ char mtext[1]; /* message data */ }; .fi .in .PP The .I mtext field is an array (or other structure) whose size is specified by .IR msgsz , a nonnegative integer value. Messages of zero length (i.e., no .I mtext field) are permitted. The .I mtype field must have a strictly positive integer value. This value can be used by the receiving process for message selection (see the description of .BR msgrcv () below). .SS msgsnd() The .BR msgsnd () system call appends a copy of the message pointed to by .I msgp to the message queue whose identifier is specified by .IR msqid . .PP If sufficient space is available in the queue, .BR msgsnd () succeeds immediately. (The queue capacity is defined by the .I msg_qbytes field in the associated data structure for the message queue. During queue creation this field is initialized to .B MSGMNB bytes, but this limit can be modified using .BR msgctl (2).) If insufficient space is available in the queue, then the default behavior of .BR msgsnd () is to block until space becomes available. If .B IPC_NOWAIT is specified in .IR msgflg , then the call instead fails with the error .BR EAGAIN . A blocked .BR msgsnd () call may also fail if: .IP * 2 the queue is removed, in which case the system call fails with .I errno set to .BR EIDRM ; or .IP * a signal is caught, in which case the system call fails with .I errno set to .BR EINTR ; see .BR signal (7). .RB ( msgsnd () is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the .B SA_RESTART flag when establishing a signal handler.) .PP Upon successful completion the message queue data structure is updated as follows: .IP .I msg_lspid is set to the process ID of the calling process. .IP .I msg_qnum is incremented by 1. .IP .I msg_stime is set to the current time. .SS msgrcv() The .BR msgrcv () system call removes a message from the queue specified by .I msqid and places it in the buffer pointed to by .IR msgp . .PP The argument .I msgsz specifies the maximum size in bytes for the member .I mtext of the structure pointed to by the .I msgp argument. If the message text has length greater than .IR msgsz , then the behavior depends on whether .B MSG_NOERROR is specified in .IR msgflg . If .B MSG_NOERROR is specified, then the message text will be truncated (and the truncated part will be lost); if .B MSG_NOERROR is not specified, then the message isn't removed from the queue and the system call fails returning \-1 with .I errno set to .BR E2BIG . .PP The argument .I msgtyp specifies the type of message requested as follows: .IP * 2 If .I msgtyp is 0, then the first message in the queue is read. .IP * If .I msgtyp is greater than 0, then the first message in the queue of type .I msgtyp is read, unless .B MSG_EXCEPT was specified in .IR msgflg , in which case the first message in the queue of type not equal to .I msgtyp will be read. .IP * If .I msgtyp is less than 0, then the first message in the queue with the lowest type less than or equal to the absolute value of .I msgtyp will be read. .PP The .I msgflg argument is a bit mask constructed by ORing together zero or more of the following flags: .TP .B IPC_NOWAIT Return immediately if no message of the requested type is in the queue. The system call fails with .I errno set to .BR ENOMSG . .TP .B MSG_EXCEPT Used with .I msgtyp greater than 0 to read the first message in the queue with message type that differs from .IR msgtyp . .TP .B MSG_NOERROR To truncate the message text if longer than .I msgsz bytes. .PP If no message of the requested type is available and .B IPC_NOWAIT isn't specified in .IR msgflg , the calling process is blocked until one of the following conditions occurs: .IP * 2 A message of the desired type is placed in the queue. .IP * The message queue is removed from the system. In this case the system call fails with .I errno set to .BR EIDRM . .IP * The calling process catches a signal. In this case the system call fails with .I errno set to .BR EINTR . .RB ( msgrcv () is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the .B SA_RESTART flag when establishing a signal handler.) .PP Upon successful completion the message queue data structure is updated as follows: .IP .I msg_lrpid is set to the process ID of the calling process. .IP .I msg_qnum is decremented by 1. .IP .I msg_rtime is set to the current time. .SH "RETURN VALUE" On failure both functions return \-1 with .I errno indicating the error, otherwise .BR msgsnd () returns 0 and .BR msgrcv () returns the number of bytes actually copied into the .I mtext array. .SH ERRORS When .BR msgsnd () fails, .I errno will be set to one among the following values: .TP .B EACCES The calling process does not have write permission on the message queue, and does not have the .B CAP_IPC_OWNER capability. .TP .B EAGAIN The message can't be sent due to the .I msg_qbytes limit for the queue and .B IPC_NOWAIT was specified in .IR msgflg . .TP .B EFAULT The address pointed to by .I msgp isn't accessible. .TP .B EIDRM The message queue was removed. .TP .B EINTR Sleeping on a full message queue condition, the process caught a signal. .TP .B EINVAL Invalid .I msqid value, or nonpositive .I mtype value, or invalid .I msgsz value (less than 0 or greater than the system value .BR MSGMAX ). .TP .B ENOMEM The system does not have enough memory to make a copy of the message pointed to by .IR msgp . .PP When .BR msgrcv () fails, .I errno will be set to one among the following values: .TP .B E2BIG The message text length is greater than .I msgsz and .B MSG_NOERROR isn't specified in .IR msgflg . .TP .B EACCES The calling process does not have read permission on the message queue, and does not have the .B CAP_IPC_OWNER capability. .TP .B EAGAIN No message was available in the queue and .B IPC_NOWAIT was specified in .IR msgflg . .TP .B EFAULT The address pointed to by .I msgp isn't accessible. .TP .B EIDRM While the process was sleeping to receive a message, the message queue was removed. .TP .B EINTR While the process was sleeping to receive a message, the process caught a signal; see .BR signal (7). .TP .B EINVAL .I msgqid was invalid, or .I msgsz was less than 0. .TP .B ENOMSG .B IPC_NOWAIT was specified in .I msgflg and no message of the requested type existed on the message queue. .SH "CONFORMING TO" SVr4, POSIX.1-2001. .SH NOTES The inclusion of .I and .I isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files. .\" Like Linux, the FreeBSD man pages still document .\" the inclusion of these header files. The .I msgp argument is declared as \fIstruct msgbuf *\fP with libc4, libc5, glibc 2.0, glibc 2.1. It is declared as \fIvoid *\fP with glibc 2.2 and later, as required by SUSv2 and SUSv3. The following limits on message queue resources affect the .BR msgsnd () call: .TP .B MSGMAX Maximum size for a message text: 8192 bytes (on Linux, this limit can be read and modified via .IR /proc/sys/kernel/msgmax ). .TP .B MSGMNB Default maximum size in bytes of a message queue: 16384 bytes (on Linux, this limit can be read and modified via .IR /proc/sys/kernel/msgmnb ). The superuser can increase the size of a message queue beyond .B MSGMNB by a .BR msgctl (2) system call. .PP The implementation has no intrinsic limits for the system wide maximum number of message headers .RB ( MSGTQL ) and for the system wide maximum size in bytes of the message pool .RB ( MSGPOOL ). .SH "SEE ALSO" .BR msgctl (2), .BR msgget (2), .BR capabilities (7), .BR mq_overview (7), .BR svipc (7) .SH COLOPHON This page is part of release 3.44 of the Linux .I man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.